This invention concerns a method of forming an oxide superconductor/semiconductor junction, and in particular, a method of forming an oxide superconductor/semiconductor junction wherein one of the constituent elements of the oxide superconductor is bismuth (referred to hereafter as Bi) or thallium (referred to hereafter as Tl), and the semiconductor is an elemental semiconductor or a Group III-V or Group II-VI compound semiconductor.
Conventionally, thin films of high temperature oxide superconductors such as Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.x, Bi.sub.2 (Sr, Ca).sub.3 Cu.sub.2 O.sub.y, or Tl.sub.2 (Ba, Ca).sub.3 Cu.sub.2 O.sub.z were formed on a substrate such as silicon (Si) by first forming a buffer layer of a substance such as MgO, ZrO.sub.2, or BaTiO.sub.3 (or SrTiO.sub.3)/MgAl.sub.2 O.sub.4 on a Si substrate, and then growing a thin film of the oxide superconductor on the buffer layer.
Methods where MgO or ZrO.sub.2 are used as the buffer layer are disclosed, for example, in the following references:
Reference 1: Japanese Journal of Applied Physics, 27 [8] (August 1988), p. L1524-1526.
Reference 2: Appl. Phys. Lett.: American Institute of Physics, 52 [24] (Jun. 13, 1988), p. 2068-2070.
A method where BaTiO.sub.3 (or SrTiO.sub.3)/MgAl.sub.2 O.sub.4 is used as said buffer layer is disclosed in the following reference:
Reference 3: Appl. Phys. Lett.: American Institute of Physics, 53 [20] (Nov. 14, 1988), p. 1967-1969.
In addition to these methods where a junction between a high temperature oxide superconductor and a semiconductor is made using a suitable buffer layer, the oxide superconductor may be deposited directly on the Si or other substrate as disclosed in Reference 4.
Reference 4: Japanese Journal of Applied Physics, 27 [12 ] (December 1988), p. L2442-2444.
The technique described in this reference is known as laser ablation vapor deposition. It consists of heating and evaporating the bulk oxide superconductor by for example an eximer laser in order to deposit it on the Si or other substrate.
The conventional methods of forming an oxide superconductor/semiconductor junction by means of a buffer layer as described in references 1, 2 and 3, were effective in devices where the thin film of oxide superconductor was used as a superconductor, but they could not be applied in techniques where the interface with the elemental or compound semiconductor was used, as for example in complex superconductor/semiconductor devices such as the superconductor-base transistor disclosed in Japanese Patent Application No. 63-132850 proposed by the applicant of this application. Further, in the laser ablation vapor deposition method described in Reference 4, a bulk material of oxide superconductor is heated and vaporized by the laser. It is thus impossible to impose any conditions on the arrangement of atoms at the interface, and this method cannot therefore be applied to devices, such as for example the above-mentioned complex superconductor/semiconductor devices, which make use of the superconductor/semiconductor interface.
In summary, according to the prior art, when a thin superconducting film was formed by conventional techniques, Si reacted with the elements of the superconductor composition at the interface so that the properties of the film deteriorated, and due to reactions at the interface, the boundary became indistinct. In particular, an element such as Si which is easily oxidized reacted with the oxygen in the high-temperature oxide superconductor when and after the thin film is formed. In other words, as thin films of conventional superconductors were formed by methods such as sputtering, it was impossible to form a junction on a substrate of an elemental semiconductor or a semiconductor of a Group III-V or Group II-VI compound with a satisfactory interface. It was thus not possible to form junctions in superconductor transistors or the like.